Android Low on memory问题

话不多说，先贴上测试提出来的bug

尽管我极力说，这个不算bug，但是人家就是不相信，只好给测试萌妹写一个分析过程了。

这句log在logcat 中的输出如下：

26780 19882 I ActivityManager: Low on memory:
26780 19882 I ActivityManager:   ntv   ??  172100: idps_sc (pid 2257) native
26780 19882 I ActivityManager:   ntv   ??   34873: surfaceflinger (pid 1643) nativ
...

说明是 ActivityManagerService打印出来的，那么我们看一下代码打印的地方

void reportMemUsage(ArrayList memInfos) {
		...省略
        tag.append("Low on memory -- ");
        appendMemBucket(tag, totalPss, "total", false);
        appendMemBucket(stack, totalPss, "total", true);
		...省略
    }

调用这个方法地方在

case REPORT_MEM_USAGE_MSG: {
    final ArrayList memInfos = (ArrayList)msg.obj;
    Thread thread = new Thread() {
        @Override public void run() {
            reportMemUsage(memInfos);
        }
    };
    thread.start();
    break;
}

REPORT_MEM_USAGE_MSG这个这个消息是在

    final void doLowMemReportIfNeededLocked(ProcessRecord dyingProc) {
    	...省略
        if (!haveBg) {
        	// 1.只有当ro.debuggable 为1的时候 doReport才为true ，大概率是在userdebug版本
            boolean doReport = "1".equals(SystemProperties.get(SYSTEM_DEBUGGABLE, "0"));
              	...省略
            }
            if (doReport) {
	            // 2. 当doReport为true的时候才会发出REPORT_MEM_USAGE_MSG 这条消息
                Message msg = mHandler.obtainMessage(REPORT_MEM_USAGE_MSG, memInfos);
                mHandler.sendMessage(msg);
            }
            scheduleAppGcsLocked();
        }
    }

而doLowMemReportIfNeededLocked这个方法的调用：
1、在killAllBackgroundProcesses里面
2、在appDiedLocked里面，在appDiedLocked中会先判断app.killedByAm，如果是am kill的，则不会report信息，但是日志中的是lmk杀死的，则会report这条信息

如果要看appDiedLocked这个方法，首先要知道在ActivityThread.main()中，从这里开始进程创建之后会从这个方法开始绑定AMS，在AMS中通过调用AMS.attachApplicationLocked()这个方法开始绑定，在这个方法中会有对ApplicationThreadProxy绑定通知，见代码：

private final boolean attachApplicationLocked(IApplicationThread thread,
        int pid) {
    //绑定死亡通知,此处thread真实数据类型为ApplicationThreadProxy
    AppDeathRecipient adr = new AppDeathRecipient(app, pid, thread);
    thread.asBinder().linkToDeath(adr, 0);
    app.deathRecipient = adr;
}

在这个过程中，当我们binder server挂掉后，会回调AppDeathRecipient.binderDied()方法通知client端。
那到这里我们的server端是指应用进程的ApplicationThread，其中ApplicationThread是在ActivityThread中创建的此时属于新建的进程（比如新建app的进程）。client就是ApplicationThreadProxy对象，这个对象是在AMS中，AMS是在system_server中。所以当我们binder server端死亡的时候（app进程死亡）我们system_server进程就会收到通知，然后做一些处理。在这盗用一波别人花的流程图：

然后我们看一下appDiedLocked这个方法

@GuardedBy("this")
    final void appDiedLocked(ProcessRecord app) {
       appDiedLocked(app, app.pid, app.thread, false);
    }

    @GuardedBy("this")
    final void appDiedLocked(ProcessRecord app, int pid, IApplicationThread thread,
            boolean fromBinderDied) {
        // First check if this ProcessRecord is actually active for the pid.
        synchronized (mPidsSelfLocked) {
            ProcessRecord curProc = mPidsSelfLocked.get(pid);
            if (curProc != app) {
                Slog.w(TAG, "Spurious death for " + app + ", curProc for " + pid + ": " + curProc);
                return;
            }
        }

        BatteryStatsImpl stats = mBatteryStatsService.getActiveStatistics();
        synchronized (stats) {
            stats.noteProcessDiedLocked(app.info.uid, pid);
        }

		//当进程还没有设置已被杀的标记,则进入该分支杀掉相应进程
        if (!app.killed) {
	        //非binder死亡回调,而是上层直接调用该方法,则进入该分支
            if (!fromBinderDied) {
                killProcessQuiet(pid);
            }
            killProcessGroup(app.uid, pid);
            app.killed = true;
        }

        // Clean up already done if the process has been re-started.
        if (app.pid == pid && app.thread != null &&
                app.thread.asBinder() == thread.asBinder()) {
            //这个地方一般为true
            boolean doLowMem = app.instr == null;
            boolean doOomAdj = doLowMem;
            //当app不是由am所杀,通常情况下都是lmk所杀
            if (!app.killedByAm) {
                reportUidInfoMessageLocked(TAG,
                        "Process " + app.processName + " (pid " + pid + ") has died: "
                                + ProcessList.makeOomAdjString(app.setAdj)
                                + ProcessList.makeProcStateString(app.setProcState), app.info.uid);
                //既然是由lmk所杀,说明当时内存比较紧张,这时希望能被杀
                mAllowLowerMemLevel = true;
            } else {
                // Note that we always want to do oom adj to update our state with the
                // new number of procs.
                mAllowLowerMemLevel = false;
                doLowMem = false;
            }
            EventLog.writeEvent(EventLogTags.AM_PROC_DIED, app.userId, app.pid, app.processName,
                    app.setAdj, app.setProcState);
            if (DEBUG_CLEANUP) Slog.v(TAG_CLEANUP,
                "Dying app: " + app + ", pid: " + pid + ", thread: " + thread.asBinder());

            /从ams移除该进程以及connections
            handleAppDiedLocked(app, false, true);

			//一般为true，则需要更新各个进程的adj
            if (doOomAdj) {
                updateOomAdjLocked();
            }
            
            //当进程是由lmkd所杀,则进入该分支
            if (doLowMem) {
	            //只有当mLruProcesses中所有进程都运行在前台,才报告内存信息
	            //mLruProcesses以LRU顺序存储了当前运行的应用程序进程信息，mLruProcesses中的第一个元素就是最近最少使用的进程对应的ProcessRecord，用于管理应用程序进程
	            //3种情况可以更新mLruProcesses：
				//1.应用程序异常退出：调用handleAppDiedLocked更新mLruProcesses
				//2.显式“杀死”指定进程：调用AMS显式“杀死”进程时需要更新mLruProcesses
				//3.启动和调度应用程序4大组件：调用updateLruProcessLocked更新mLruProcesses
				// 应用程序异常退出后，其进程也会被“杀死”。因此1、2情况都是从mLruProcesses列表中删除该进程信息。
                doLowMemReportIfNeededLocked(app);
            }
        } else if (app.pid != pid) {
            // A new process has already been started.
            reportUidInfoMessageLocked(TAG,
                    "Process " + app.processName + " (pid " + pid
                            + ") has died and restarted (pid " + app.pid + ").", app.info.uid);
            EventLog.writeEvent(EventLogTags.AM_PROC_DIED, app.userId, app.pid, app.processName);
        } else if (DEBUG_PROCESSES) {
            Slog.d(TAG_PROCESSES, "Received spurious death notification for thread "
                    + thread.asBinder());
        }

        // On the device which doesn't have Cgroup, log LmkStateChanged which is used as a signal
        // for pulling memory stats of other running processes when this process died.
        if (!hasMemcg()) {
            StatsLog.write(StatsLog.APP_DIED, SystemClock.elapsedRealtime());
        }
    }

我们再看上述方法中handleAppDiedLocked()这个方法，这个方法就是要从AMS中移除这个进程的信息以及一些组件信息

    /**
     * Main function for removing an existing process from the activity manager
     * as a result of that process going away.  Clears out all connections
     * to the process.
     */
    @GuardedBy("this")
    private final void handleAppDiedLocked(ProcessRecord app,
            boolean restarting, boolean allowRestart) {
        int pid = app.pid;
        final boolean clearLaunchStartTime = !restarting && app.removed && app.foregroundActivities;

        //清理应用程序servcie，content providers，BroadcastReceiver
        boolean kept = cleanUpApplicationRecordLocked(app, restarting, allowRestart, -1,
                false /*replacingPid*/);

        //这里主要看这个进程信息是不是还需要保存，如果不需要保存，并且不需要重启
	    //则会在lruProcess中去除这个进程信息，并且在ProcessList中移除掉
        if (!kept && !restarting) {
            removeLruProcessLocked(app);
            if (pid > 0) {
                ProcessList.remove(pid);
            }
        }

        if (mProfileProc == app) {
            clearProfilerLocked();
        }

        // Remove this application's activities from active lists.
        //清理activity相关信息, 当应用存在可见的activity则返回true
        boolean hasVisibleActivities = mStackSupervisor.handleAppDiedLocked(app);

        app.clearRecentTasks();

        app.activities.clear();

        if (app.instr != null) {
            Slog.w(TAG, "Crash of app " + app.processName
                  + " running instrumentation " + app.instr.mClass);
            Bundle info = new Bundle();
            info.putString("shortMsg", "Process crashed.");
            finishInstrumentationLocked(app, Activity.RESULT_CANCELED, info);
        }

        mWindowManager.deferSurfaceLayout();
        try {
	        //当死亡的app存在可见的Activity, 则恢复栈顶第一个非finish的activity
            if (!restarting && hasVisibleActivities
                    && !mStackSupervisor.resumeFocusedStackTopActivityLocked()) {
                //恢复top activity失败,则再次确保有可见的activity
                mStackSupervisor.ensureActivitiesVisibleLocked(null, 0, !PRESERVE_WINDOWS);
            }
        } finally {
            mWindowManager.continueSurfaceLayout();
        }
	//......
    }

先通过cleanUpApplicationRecordLocked()清除应用里面有关service, BroadcastReceiver, ContentProvider的记录相关，然后清理activity相关信息，当应用存在可见的activity则返回true尝试进行恢复，注意有个参数allowRestart

接下来我们看在appDiedLocked中调用的的cleanUpApplicationRecordLocked方法，这个方法有点长

    private final boolean cleanUpApplicationRecordLocked(ProcessRecord app,
            boolean restarting, boolean allowRestart, int index, boolean replacingPid) {
        if (index >= 0) {
            removeLruProcessLocked(app);
            ProcessList.remove(app.pid);
        }

        mProcessesToGc.remove(app);
        mPendingPssProcesses.remove(app);
        ProcessList.abortNextPssTime(app.procStateMemTracker);
//  ------------------------清除crashing的弹框------------------------------
    //如果存在，则清除crash/anr/wait对话框
        // Dismiss any open dialogs.
        if (app.crashDialog != null && !app.forceCrashReport) {
            app.crashDialog.dismiss();
            app.crashDialog = null;
        }
        if (app.anrDialog != null) {
            app.anrDialog.dismiss();
            app.anrDialog = null;
        }
        if (app.waitDialog != null) {
            app.waitDialog.dismiss();
            app.waitDialog = null;
        }

        app.crashing = false;
        app.notResponding = false;
    ------------------------做一些基本的清理工作------------------------------
        app.resetPackageList(mProcessStats);  //重置包列表
        app.unlinkDeathRecipient();	//接触死亡回调
        app.makeInactive(mProcessStats);
        app.waitingToKill = null;
        app.forcingToImportant = null;
        updateProcessForegroundLocked(app, false, false);	//将app从前台进程移除
        app.foregroundActivities = false;
        app.hasShownUi = false;
        app.treatLikeActivity = false;
        app.hasAboveClient = false;
        app.hasClientActivities = false;
 ------------------------清理service------------------------------
        mServices.killServicesLocked(app, allowRestart);

        boolean restart = false;
------------------------清理ContentProvider------------------------------
        // Remove published content providers.
        for (int i = app.pubProviders.size() - 1; i >= 0; i--) {
	        //获取该进程已发表的ContentProvider
            ContentProviderRecord cpr = app.pubProviders.valueAt(i);
            final boolean always = app.bad || !allowRestart;
            //ContentProvider服务端被杀，则client端进程也会被杀
            boolean inLaunching = removeDyingProviderLocked(app, cpr, always);
            if ((inLaunching || always) && cpr.hasConnectionOrHandle()) {
                // We left the provider in the launching list, need to
                // restart it.
                restart = true;
            }

            cpr.provider = null;
            cpr.proc = null;
        }
        app.pubProviders.clear();

        // Take care of any launching providers waiting for this process.
        //处理正在启动并且是有client端正在等待的ContentProvider
        if (cleanupAppInLaunchingProvidersLocked(app, false)) {
            restart = true;
        }

        // Unregister from connected content providers.
        //取消已连接的ContentProvider的注册
        if (!app.conProviders.isEmpty()) {
            for (int i = app.conProviders.size() - 1; i >= 0; i--) {
                ContentProviderConnection conn = app.conProviders.get(i);
                conn.provider.connections.remove(conn);
                stopAssociationLocked(app.uid, app.processName, conn.provider.uid,
                        conn.provider.name);
            }
            app.conProviders.clear();
        }

        // At this point there may be remaining entries in mLaunchingProviders
        // where we were the only one waiting, so they are no longer of use.
        // Look for these and clean up if found.
        // XXX Commented out for now.  Trying to figure out a way to reproduce
        // the actual situation to identify what is actually going on.
        if (false) {
            for (int i = mLaunchingProviders.size() - 1; i >= 0; i--) {
                ContentProviderRecord cpr = mLaunchingProviders.get(i);
                if (cpr.connections.size() <= 0 && !cpr.hasExternalProcessHandles()) {
                    synchronized (cpr) {
                        cpr.launchingApp = null;
                        cpr.notifyAll();
                    }
                }
            }
        }
  ------------------------清理BroadcastReceiver------------------------------
        skipCurrentReceiverLocked(app);

        // Unregister any receivers.
        // 取消注册的广播接收者
        for (int i = app.receivers.size() - 1; i >= 0; i--) {
            removeReceiverLocked(app.receivers.valueAt(i));
        }
  ---------------------------清理process------------------------------
        app.receivers.clear();

        // If the app is undergoing backup, tell the backup manager about it
        if (mBackupTarget != null && app.pid == mBackupTarget.app.pid) {
            if (DEBUG_BACKUP || DEBUG_CLEANUP) Slog.d(TAG_CLEANUP, "App "
                    + mBackupTarget.appInfo + " died during backup");
            mHandler.post(new Runnable() {
                @Override
                public void run(){
                    try {
                        IBackupManager bm = IBackupManager.Stub.asInterface(
                                ServiceManager.getService(Context.BACKUP_SERVICE));
                        bm.agentDisconnected(app.info.packageName);
                    } catch (RemoteException e) {
                        // can't happen; backup manager is local
                    }
                }
            });
        }

        for (int i = mPendingProcessChanges.size() - 1; i >= 0; i--) {
            ProcessChangeItem item = mPendingProcessChanges.get(i);
            if (app.pid > 0 && item.pid == app.pid) {
                mPendingProcessChanges.remove(i);
                mAvailProcessChanges.add(item);
            }
        }
        mUiHandler.obtainMessage(DISPATCH_PROCESS_DIED_UI_MSG, app.pid, app.info.uid,
                null).sendToTarget();

        // If the caller is restarting this app, then leave it in its
        // current lists and let the caller take care of it.
        if (restarting) {
            return false;
        }

        if (!app.persistent || app.isolated) {
            if (DEBUG_PROCESSES || DEBUG_CLEANUP) Slog.v(TAG_CLEANUP,
                    "Removing non-persistent process during cleanup: " + app);
            if (!replacingPid) {
                removeProcessNameLocked(app.processName, app.uid, app);
            }
            if (mHeavyWeightProcess == app) {
                mHandler.sendMessage(mHandler.obtainMessage(CANCEL_HEAVY_NOTIFICATION_MSG,
                        mHeavyWeightProcess.userId, 0));
                mHeavyWeightProcess = null;
            }
        } else if (!app.removed) {
            // This app is persistent, so we need to keep its record around.
            // If it is not already on the pending app list, add it there
            // and start a new process for it.
            //对于persist的应用就重启一下
            if (mPersistentStartingProcesses.indexOf(app) < 0) {
                mPersistentStartingProcesses.add(app);
                restart = true;
            }
        }
        if ((DEBUG_PROCESSES || DEBUG_CLEANUP) && mProcessesOnHold.contains(app)) Slog.v(
                TAG_CLEANUP, "Clean-up removing on hold: " + app);
        //mProcessesOnHold：记录着试图在系统ready之前就启动的进程。
	    //在那时并不启动这些进程，先记录下来,等系统启动完成则启动这些进程。
        mProcessesOnHold.remove(app);

        if (app == mHomeProcess) {
            mHomeProcess = null;
        }
        if (app == mPreviousProcess) {
            mPreviousProcess = null;
        }

        if (restart && !app.isolated) {
            // We have components that still need to be running in the
            // process, so re-launch it.
            /仍有组件需要运行在该进程中，因此重启该进程
            if (index < 0) {
                ProcessList.remove(app.pid);
            }
            addProcessNameLocked(app);
            app.pendingStart = false;
            startProcessLocked(app, "restart", app.processName);
            return true;
        } else if (app.pid > 0 && app.pid != MY_PID) {
            // Goodbye!
            //移除该进程相关信息
            boolean removed;
            synchronized (mPidsSelfLocked) {
                mPidsSelfLocked.remove(app.pid);
                mHandler.removeMessages(PROC_START_TIMEOUT_MSG, app);
            }
            mBatteryStatsService.noteProcessFinish(app.processName, app.info.uid);
            if (app.isolated) {
                mBatteryStatsService.removeIsolatedUid(app.uid, app.info.uid);
            }
            app.setPid(0);
        }
        return false;
    }

然后吧，我就感觉扯远了。。。